The X-Ray Scattering Halo Around Nova Cygni 1992: Testing a Model for Interstellar Dust

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  • 15 August 2002
Abstract
We use published ROSAT observations of the X-ray Nova V1974 Cygni 1992 to test a model for interstellar dust grains consisting of a mixture of carbonaceous grains and silicate grains. The time-dependent X-ray emission from the nova is modelled, and X-ray scattering is calculated for a dust mixture with a realistic size distribution. Model results are compared with the scattering halos measured by ROSAT at 9 different epochs. We find that the observed X-ray halos appear to be consistent with the halos calculated for the size distribution of Weingartner & Draine which reproduces the Milky Way extinction with R_V=3.1, provided that the reddening to the nova is E(B-V) approx 0.19, consistent with E(B-V)=0.19 inferred from the late-time Balmer decrement. The time delay of the scattered halo relative to the direct flux from the nova is clearly detected. Models with smoothly-distributed dust give good overall agreement with the observed scattering halo, but tend to produce somewhat more scattering than observed at 200--300", and insufficient scattering at 50--100". While an additional population of large grains can increase the scattered intensity at 50--100", this could also be achieved by having ~20% of the dust in a cloud at a distance from us equal to ~90% of the distance to the nova. The observations therefore do not require a population of micron-sized dust grains. Future observations by Chandra of X-ray scattering halos around point sources can provide more stringent tests of interstellar dust models.

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